A scanning probe microscope (SPM) is a tool that enables formation of an image of a surface with angstrom-scale vertical and lateral resolution. There are many types of SPMs, including atomic-force microscopes (AFMs), ballistic electron emission microscopes (BEEMs), scanning thermal microscopes (SThMs), scanning tunneling microscopes (STMs), and many more. Each of these SPM types interrogates a surface and/or scans the surface in a different manner; however, in each, a probe tip is scanned over the surface and records the value of a surface parameter at myriad discrete locations. These parameter values are then mapped to form the image of the surface.
Dynamic AFM is, by far, the most widely used modality of scanning probe microscopy. In dynamic AFM, a sharp probe tip is moved very close to the surface under study (typically within Angstroms) and the probe is then raster scanned across the surface. The probe tip is connected to a resonator that is driven into oscillation during measurement. An image of the surface is then constructed from the force interactions between the tip and the surface during the scan. Dynamic AFM may be performed in the non-contact regime (in which tip-sample interaction forces are strictly attractive) or in the intermittent contact regime, in which the tip-sample interaction forces may be repulsive. If the probe is not operated in resonance, the modality is known as contact AFM, or static AFM. Contact AFM operates in the repulsive regime.
The force interactions between the tip and surface are determined using one of two common techniques—amplitude-modulation AFM (AM-AFM) or frequency-modulation AFM (FM-AFM). In AM-AFM, the force interactions between the tip and surface are determined by measuring the change in amplitude of the oscillation at a constant frequency—typically, just off the resonant frequency of the cantilever. In FM-AFM, the change in frequency of an oscillator that incorporates the resonator is measured using a feedback circuit (usually using a phase-locked loop), while the sensor is driven at its characteristic resonance. In cases where the cantilever is not in resonance, its static deflection can be used to determine tip-sample interaction forces; however, at significantly reduced sensitivity.
Typically, prior-art SPMs rely on the use of piezoelectric actuators. Unfortunately, piezoelectric actuators have several drawbacks with respect to their use in these applications. Piezoelectric actuators are relatively large but provide only a small range of motion. Their large size leads to bulky scanner designs that are susceptible to thermal drift, external vibration, and shock. In addition, piezoelectric actuators notoriously exhibit creep and hysteresis that, in an SPM, manifests as image distortion.
The large size and mass of piezoelectric actuator-based SPMs also limits mechanical bandwidth, which, in turn, limits imaging rate. Arrays of SPMs for performing distributed measurements have been suggested for alleviating the bandwidth bottleneck. Unfortunately, as arrays are scaled upwards, scanning speed declines to accommodate larger payloads.
MEMS-based thermal actuators offer the promise of overcoming some of the drawbacks of piezoelectric actuators for SPM applications, since they can be small yet still generate considerable force with a large range of motion. In addition, a thermal actuator can respond quickly to a control signal. Unfortunately, prior-art thermal actuators also have significant drawbacks that limit their utility in SPM applications.
First, many thermal actuators have a tendency to buckle out-of-plane when operated in a compressive mode.
Second, the response of a conventional thermal actuator is typically controlled via Joule heating that is induced by passing electric current through the device. In such operation, however, the temperature distribution within the device is difficult to control.
Third, thermal actuators are normally fabricated using low-stress material systems, such as Silicon-on-Insulator, where the structural material of the device comprises the substantially zero-stress silicon active-layer material. This avoids the high residual stress, as well as stress gradients that normally characterize thin films deposited on a substrate. As a result, most MEMS-based thermal actuators are poorly suited to fabrication using conventional high-volume foundry processes such as CMOS fabrication. The inability to fabricate actuators in a CMOS foundry, for example, impairs the ability to integrate thermal actuators with high-performance electronic circuitry.
Another challenge for prior-art SPM applications is measuring the tip-sample interaction forces with high bandwidth and resolution. Typically, these forces are measured using external optics (e.g., a laser-based displacement sensor) having a large free-space path. Such optical systems tend to be quite large, in and of themselves. In addition, they are expensive and cumbersome to align, adding significantly to the high cost of state-of-the-art SPM systems. Further, their large size and expense makes them difficult to implement in arrayed SPM systems.
Small, preferably single-chip, inexpensive, fast, stable and independent SPMs that do not incur bandwidth penalties upon array scaling would, therefore, be a significant advance in the state of the art.